1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a two-dimensional magnetic recording (TDMR) HDD.
2. Description of the Related Art
HDDs typically have disks with data tracks with a fixed track spacing or pitch that is set during manufacturing and cannot be changed during the life of the HDD. The positioning of the read/write heads to the data tracks is accomplished by servo tracks that have angularly-spaced servo sectors that contain head positioning information. The servo sectors extend radially across the data tracks. The read head detects the positioning information as the disk rotates and passes the position information to a servo control system to maintain the head on the desired data track. The servo tracks also have a fixed track pitch that is set during manufacturing and that may be different from the fixed track pitch of the data tracks.
A proposed HDD technology that uses multiple read heads or sensors is two-dimensional magnetic recording (TDMR). In TDMR, multiple sensors that are located on a single head structure access the same or adjacent data tracks to obtain signals that are processed jointly. This allows the data tracks to be placed closer together, resulting in an increase in areal data bit density. A head structure with multiple stacked read sensors for TDMR is described in US 2013/0286502 A1 and in U.S. Pat. No. 8,873,204 B1 which is assigned to the same assignee as this application.
It is important in TDMR that the read heads accurately follow the servo tracks to avoid track misregistration (TMR) during reading and writing. A reduction in TMR in a TDMR disk drive depends on the number of servo half-tracks that are being read by the multiple sensors during reading and writing. The servo tracks are written in half-track increments in the disk drive or by a dedicated servowriter during disk drive manufacturing. However, during the servowriting process, media noise, write head motion and mechanical disturbances cause a written-in track shape error for each half-track. However, if all of the sensors are reading different servo half-tracks the noise from multiple servo half-tracks can be averaged out and TMR will be reduced. The number of servo half-tracks that can be followed in a multiple read head structure is a function of the servo track pitch and the cross-track sensor-to-sensor spacing (CTS), which in turn is a function of a factor referred to as head “skew”.
Skew arises because the sensors are supported on a radial actuator that causes the sensors to make an arcuate path across the disk. At the mid-diameter (MD) regions of the disk the skew angle θ (the angle between a line orthogonal to the sensor and the servo track) is near zero. However, at the ID and OD regions the skew angle can be up to 15-20 degrees, depending on the geometry of the actuator and disk. As the skew angle increases the CTS of the multiple sensors also increases.
What is needed is a TDMR HDD that compensates for head skew and thus allows the multiple read heads to follow multiple servo half-tracks so as to reduce TMR.